Electrodes which are used to detect and measure the amount of fluoride ion in a liquid medium (e.g., water) utilize a crystal membrane retained in one end of an elongated barrel. The electrode comprises a crystal membrane, a housing, an internal filling solution, a silver chloride wire, a cap, and a cable terminating normally with a BNC connector.
The fluoride electrode, connected with a reference electrode and placed in a liquid sample, generates a voltage signal which is proportional to the fluoride concentration in the sample. The crystal membrane in the probe is in contact with the internal filling solution and an external sample solution.
The filling solution contains set concentrations of chloride and fluoride ions. The chloride ions establish a voltage with the internal silver/silver chloride reference element and the fluoride ions establish an equilibrium and potential across the membrane. This potential varies only with the fluoride ion concentration (activity) in the sample.
The reference electrode, which is required to complete the electrical circuit through a pH/mV meter, establishes a stable, reproducible potential. In the completed cell, only the potential across the crystal membrane varies.
The crystal membrane preferably comprises lanthanum fluoride doped with europium fluoride (e.g., 0.3%). Alternatively, the crystal membrane may comprise cerium fluoride doped with europium fluoride. The size of the membrane may vary. Preferably the exposed surface of the membrane is flat and polished.
In the conventional electrode, the cylindrical crystal membrane is secured in a cylindrical plastic cup by means of a potting compound (bonding agent) which is very expensive. The process of securing the crystal is time-consuming, expensive and unreliable. The plastic cup must then be bonded into an elongated plastic housing. The presence of any gaps or delamination between the crystal membrane and the potting compound (either during manufacture or after the electrode has been used) allows the internal filling solution in the electrode to contact the sample solution, thereby creating a current path around the crystal and rendering the electrode inaccurate. More critical delamination creates a short circuit, rendering the electrode useless. Eventually the crystal can fall out of the electrode.
There has not heretofore been provided a method or technique for effectively and easily securing the crystal in the electrode.